Arduino UNO Project

Hy there,

I am kinda new here and I want to start a new project based on Arduino UNO. I want to control 3 leds or 3 relays( I don't know so far) from my computer via USB->Arduino UNO. I have a C program that when runs write a value in a .txt file. That value I want to be like a command for the uC: what led should turn ON.

Its something I can do with Arduino UNO?

Arduino can not read files from the PC, you must write any program for PC to sendi commands via serial line (COMx) to Arduino.

Hmmm...how can I do that?